The present invention relates to a dynamic pressure bearing, and more particularly to a dynamic pressure bearing which can easily cope with increasing and decreasing of a lubricant, in which leakage of a lubricant fluid to the outside caused by thermal expansion due to temperature rise can be surely prevented from occurring, and which can be processed relatively easily.
Recently, a dynamic pressure bearing is used more frequently as a bearing in a rotation support portion for an optical or magnetic disk drive for a computer system, or for a spindle motor. Such a dynamic pressure bearing is configured in the following manner. Herringbone-like or spiral dynamic pressure grooves are formed on one of a shaft and a sleeve which are relatively rotated and opposed via a small gap in a radial direction, or one of a flange which is formed integrally with a shaft and a sleeve in which a space for allowing the flange to be fitted thereinto via a small gap is formed. A lubricant fluid is filled into the gap, so that a non-contact bearing in a radial or thrust direction is formed by using a dynamic pressure generated during relative rotation. In such a bearing, a countermeasure is taken so that the lubricant fluid hardly leaks to the outside even when so-called “precession movement” occurs between the shaft and the sleeve at starting or stopping.
FIG. 5 shows an example of a dynamic pressure bearing in which a countermeasure for preventing a lubricant fluid from leaking is taken. The exemplified bearing is a radial dynamic pressure bearing in which dynamic pressure grooves 13 are formed in an inner peripheral face of a sleeve 12 that is relatively rotated, and a shaft 11 is fitted into the sleeve. In the radial dynamic pressure bearing, a tapered portion 11a in which the diameter is smaller as further moving toward the open side is disposed in an end part of the shaft 11, and a shaft portion 11c in which a step 11b is formed to have the same shaft diameter as that of the inner portion is disposed on an axially outer side of the tapered portion 11a. The dynamic pressure bearing is configured so that a recess 14 for storing a lubricant fluid G is formed in the inner diameter portion of the sleeve 12 surrounding the shaft portion in which the tapered portion 11a is disposed, an open side of the recess 14 is narrowed by the shaft portion 11c, and the lubricant fluid G is pulled in toward the dynamic pressure bearing by the capillary effect due to the capillary action, thereby preventing the fluid from leaking.
Another thrust dynamic pressure bearing in which, as shown in FIG. 6, a flange 21f disposed on a shaft 21 is fitted into a recess 22a formed in a sleeve 22, dynamic pressure grooves 21c are formed on a face 21b of the flange 21f perpendicular to the axial direction to form a dynamic pressure bearing 23 between the face and an opposing face 22b of the sleeve 22 is known (Japanese Patent Publication (Kokai) No. HEI9-273543). In the bearing, an annular narrow passage 24 communicating with the recess 22a is formed between a peripheral face 21a of the shaft 21 and an inner peripheral face 22c of the sleeve 22, and a lubricant fluid reservoir 25 configured by a truncated conical inner peripheral face 22d is formed in a position which is slightly separated toward the outside from the annular narrow passage 24.
A spindle motor shown in FIG. 7 is known (Japanese Patent Publication (Kokai) No. 2000-192946). In the spindle motor, a hub 32 serving as a rotating member of the spindle motor is supported by a shaft 31 serving as a stationary member, via a radial hydrodynamic bearing R and a thrust hydrodynamic bearing S. The spindle motor comprises a flange 31f constituting a thrust receiving surface 31a (31b) of the thrust hydrodynamic bearing S, and a thrust cover 34 constituting a thrust bearing surface 33 which is opposed to the flange. The thrust cover 34 is configure by a thrust plate 35 which receives a load, a seal plate 36 which is separated from the thrust plate 35 in the axial direction, and a pocket gap 37 which is interposed between the plates 35 and 36 to hold a leaking lubricant fluid, and which has a V-like section shape.
Another dynamic pressure bearing in which a lubricant fluid reservoir is disposed in a lower portion of the bearing is known (Japanese Patent Publication (Kokai) No. 2001-82458). As shown in FIG. 8, the dynamic pressure bearing is configured by a housing 42, and a shaft 41 which is fitted into the housing 42 with forming a constant gap therebetween. Dynamic pressure grooves are formed in one of an outer peripheral face of the shaft 41 and an inner peripheral face of the housing 42. A tapered shaft portion 41a in which the diameter is smaller as further moving toward the open side is formed in the shaft 41, and a similar tapered inner peripheral face 42a is formed in a position where the shaft portion 41a is placed, to form a constant space 45, thereby forming a capillary sealing portion 43 in which the inner diameter of the tapered inner peripheral face 42a of the housing 42 is gradually increased as moving from an opening 42a toward the inner area in the axial direction.
As described above, in a dynamic pressure bearing in which an opening portion is formed on the atmospheric side, a gap shape of opposing bearing portions in the interface between a lubricant fluid and the air (atmosphere) is provided with a tapered portion in which the diameter is smaller as further moving in a direction opposite to a centrifugal force, i.e., toward the open side. Furthermore, constant “wettability” is required between the tapered shaft surface and the lubricant fluid. In order to ensure the “wettability”, a certain surface roughness must be attained in a process of processing the surface. However, a fine finishing process must be performed because a stepped portion is in a gap space in a lubricant fluid reservoir. In such a case, therefore, it is difficult to balance the finishing accuracy with the surface roughness. A fine tapering process which is to be applied to a hard shaft causes the process cost to be increased.
On the other hand, during an operation of such a dynamic pressure bearing, the temperature of a lubricant fluid is raised, and the volume of the lubricant fluid in the lubricant fluid reservoir in the vicinity of the opening is increased, so that the lubricant fluid may leak to the outside during “precession movement” which occurs at starting, stopping, or the like. By contrast, at ordinary temperature, a given amount of lubricant fluid must be held in the lubricant fluid reservoir. Therefore, the capacity of the lubricant fluid reservoir space on the side of the opening must be formed so as to have a capacity which can cope with increasing and decreasing of the lubricant fluid.
In the formation of the space for holding the lubricant fluid (the lubricant fluid reservoir), consideration must be taken not only on that the lubricant fluid is held so as not to leak, but also on that the capillary effect is maintained so as to exert the function of rapidly pulling back the lubricant fluid to the dynamic pressure bearing portion. Consequently, it is difficult to determine the shape of the space.